The electrolytic solution for lithium batteries is extremely incompatible with water. Even a trace amount of water causes the problems such as inactivating the surface of a lithium negative electrode by reacting therewith to form an oxide film, and raising the internal impedance of a battery by forming air bubbles. For this reason, it is important to sufficiently perform a refining dehydration of an electrolytic solution for the improvement of battery quality.
The method for dehydrating a composition comprising lithium bis(pentafluoroethanesulfonyl)imide as an electrolyte is not well known heretofore. JP6-23209A discloses a method where a bis-sulfonimide salt having a C1-C4 perfluoroalkyl group of atoms is mixed with an inert fluorochemical to volatilize water and the inert chemical. In order to apply this method to lithium bis(pentafluoroethanesulfonyl)imide, we attempted the dehydration using perfluorohexane as an inert fluorochemical. However, the water obtained from the salt was at a level of 1000 ppm and the satisfactory result was not achieved. Moreover, this method is not economically excellent due to the use of an expensive inert fluorochemical.
Disclosed in JP10-338653A is a method for dehydrating a solution of lithium bis(trifluoromethylsulfonyl)imide, which is an analogous compound of lithium bis(pentafluoroethanesulfonyl)imide, wherein a dry inert gas is brought into contact therewith to evaporate water. The method needs circulation of the generated aqueous inert gas for dry through a dehydration system. In order to dehydrate the gas sufficiently by cooling it is necessary to operate a compressor and a refrigerator over a long time, which is not exactly economical.
As a method for producing a nonaqueous electrolytic solution for batteries, JP58-28174A discloses a method where a solvent and a solute are dissolved before water is removed as a part of the solvent being removed by distillation under a reduced pressure.
The publication indicates as an example the dehydration method by dissolving lithium perchlorate in propylene carbonate and removing propylene carbonate by distillation. However, the water contents per perchlorate when evaporating 10% and 20% amount of solvent are both remained at 100 ppm, which shows the limit of this method. Moreover, what should be done for the more effective dehydration is not suggested concretely.
The object of the present invention is to completely dehydrate a composition by a simple method, wherein said composition comprises lithium bis(pentafluoroethanesulfonyl)imide whose water content is difficult to be reduced due to its high deliquescence.